This article reports on the development of a neutron absorption tracer (NAT) technique for evaluating the hydrodynamic properties in direct coal liquefaction reactors. The plant was constructed at Kashima, Japan, based on the concept of the NEDOL process, and possessed three reactors which were connected in series. The reactors were 1,0 m in diameter and 11.8 in in height, and could treat 150 ton coal/day, In order to determine the residence time distribution (RTD) data of the liquid phase in the reactors, Cf-252 was used as the neutron source, and a fine gadolinium oxide powder was used as the tracer. The tracer slurry was injected into the inlet pipe of the first or third reactor, and the response was detected at the outlet pipe of the first and third reactors. Preliminary tests were performed to obtain the design principles for the neutron howitzer and the instrument system. Low- and middle-energy neutrons were more effective than thermal neutrons for the detection of the RTD of the liquid phase. The tracer particles were estimated to behave in the same manner as the liquid under conditions of coal liquefaction. The tracer injection tests were then performed 64 times during the pilot plant (PP) operation. The accuracy of the NAT technique was confirmed by comparing the gas holdups calculated from the RTD data by the NAT with those directly determined using a shutdown technique of the recycle gas, The RTD data suggested that the flow in the reactors was classified in the homogeneous bubble flow, These results show the applicability and effectiveness of the NAT technique. (C) 2002 Elsevier Science B.V. All rights reserved.